The Medusa Nebula
Braided, serpentine filaments of glowing gas suggest this nebula's popular name,
The Medusa Nebula. Also known as Abell 21, this Medusa is an old planetary nebula
some 1,500 light-years away in the constellation Gemini. Like its mythological namesake,
the nebula is associated with a dramatic transformation. The planetary nebula phase
represents a final stage in the evolution of low mass stars like the sun, as they
transform themselves from red giants to hot white dwarf stars and in the process shrug
off their outer layers. Ultraviolet radiation from the hot star powers the nebular glow.
The Medusa's transforming hot central star is visible in the detailed color image as the
small blue star within the upper half of the overall bright crescent shape. Fainter
filaments clearly extend above and to the left of the bright crescent region. The Medusa
Nebula is estimated to be over 4 light-years across.